DC power supplies energized by the AC power lines must convert the input AC voltage to a desired DC voltage level; this conversion can be accomplished in either an active manner or a passive manner. In either approach, it is desirable to achieve both a high input power factor (PF), and reduced total harmonic distortion of the AC input current. The non-linearities introduced with diode rectification of an AC source produce waveform distortion and harmonic current generation in the source current. These harmonic currents reduce the effective power factor to a value less than unity. A filter containing only passive components (e.g. inductors and capacitors) maybe employed to circulate these harmonic currents within the power supply, thereby increasing the power factor to approach unity.
The active conversion approach allows use of active power factor correction. Here, the AC input voltage is again rectified, but active switching is used and is controlled to improve the input current waveform, thus increasing the power factor. Traditionally, the input current waveform is caused to follow the input voltage waveform, by use of a separate feedback loop; the most common topology for active PF correction is the so-called `boost` configuration, due to its continuous input current characteristics and ability to cause current flow over the full cycle of the input. The boost topology does have several undesirable attributes, among which are the need for the output voltage to exceed the peak of the input voltage, thus requiring a post regulator.
A relatively new PF correction topology is the so-called `buck+boost` (or B+B) configuration which has several advantages over the `boost` configuration, including a single stage power conversion and wider input/output voltage range. Yet, hitherto available B+B convertors generally included a power-factor-correcting input current feedback loop in addition to an output voltage/current feedback loop to provide output regulation, and required a mechanism to sense current flow through the switching inductance to provide current mode control, in addition to the input current sensor required for input current control.
It is desirable to provide a buck+boost Ac-to-DC power convertor with a power factor enhancement control circuit achieving the aforementioned 0.95+ PF and reduced total harmonic distortion, while facilitating low switching frequency operation. Such a PF-corrected convertor may find particular usage in an electrically operated vehicle, which is becoming of increasing interest because of the perceived decrease in emissions by comparison with internal-combustion vehicles, and which is commonly supplied with electrical energy from batteries which have to be periodically recharged by DC voltages obtained from an AC-line-operated, high-PF power supply.